A STUDY OF THE DYNAMICS OF PD OXIDATION AND PDO REDUCTION BY H-2 AND CH4

Palladium is one of the most active catalysts for the catalytic combus tion of methane. Since Pd is oxidized during methane combustion to PdO and PdO is required for high activity, it is of interest to understan d the dynamics of Pd oxidation and the structure of the oxide formed a s well as the dynamics of PdO reduction by H-2 and CH4. In the present study isothermal and temperature-programmed oxidation and reduction w ere used to probe the dynamics of the oxidation and reduction of zirco nia-supported Pd. During uptake in oxygen, a monolayer of oxide is gen erated immediately, and upon further oxidation, the oxide forms a shel l around a core of metal with thicknesses that increase with the oxida tion temperature. The initial oxide is amorphous and subsequently tran sforms to crystalline PdO. The dynamics of Pd oxidation suggest that o xidation follows the Cabrera-Mott theory. Reduction of PdO by H-2 occu rs in a shellwise manner, consistent with a shrinking core mechanism, while reduction in CH4 occurs via an autocatalytic, nucleation mechani sm. In the latter case, small particles of Pd must first be formed on which CH4 can dissociate. The fragments (H and CHx (x = 3-1)) diffuse to the metal-oxide boundary where reduction of the oxide occurs. Consi stent with this picture, the kinetics of PdO reduction are first order in Pd initially, but then they become zero Order in Pd. (C) 1998 Acad emic Press.